The transportability of a portable or so-called "laptop" computer is, of course, related to its overall size and weight. In recent years, there has been a drive on the part of manufacturers to produce computers with ever-decreasing weight, while maintaining or increasing computing power. In fact, there has been a remarkable rate of progress in the reduction in size and weight, and the increase in computing speed and information storage capabilities of laptop computers. Ever-decreasing overall size has resulted in calling the most recent hand held products "notebook" computers.
A significant part of the overall weight of a computer is attributable to its power source or battery. While batteries that weighed a number of pounds were acceptable heretofore, the weight of present batteries must now be measured in ounces, rather than in pounds. Last generation's battery alone weighed more than this generation's overall computer, including battery. At this time, it is desirable to manufacture notebook computers weighing approximately six pounds, no more than one pound of which is attributable to the power source.
The size of a battery generally dictates its electrical storage capacity. Therefore, reduced battery size results in reduced power storage. A typical battery may operate a computer system only up to two hours if used continually. A reduction in the power consumption of a system extends the amount of time the system can be battery powered before the battery must be replaced or recharged. Over the last few decades, extremely rapid progress in the miniaturization and integration of digital electronics has resulted in considerable reduction in the power consumption of digital circuitry.
Although the size and weight of portable computers continues to decrease, a full panoply of features must be supported. Such features include a hard disk drive, a floppy disk drive, a display, a keyboard and a processor. To one extent or another, each of these components requires eletrical power. While conventional approaches to saving energy often require one or more of the components to shut down when not being used, it has been found that as a practical matter, human users require certain components (e.g., the display) to be activated continually while in use.
In portable computers, and in particular, notebook computers, it is also desirable to reduce the amount of heat that must be dissipated. Heat generation is minimized by the use of CMOS logic components which use power only while switching, as opposed to TTL or ECL logic components that continually draw power. Nevertheless, heat generation is unavoidable. In order to dissipate this heat, conventional components such as heat sinks and cooling fans have been used to maintain an operable temperature. Not only do such devices add to the overall size and weight of the computer, but devices such as cooling fans also draw on the System battery, further reducing the period of time the battery can maintain a charge.
Power consumption and heat generation can be reduced by decreasing the speed of the system clock. The speed of all processor functions, such as the foreground and background processes, and the bandwidth of the I/O of the processor are proportional to the system clock speed. The reduced system clock speed approach is problematic since lower clock speeds result in poorer performance; the time required to complete an information processing task is proportional to the clock speed. In order to give users the best possible performance, the system must be run at the highest possible speed. Furthermore, background processes, such as communications programs and fax spoolers, may fail with reduced clock speeds.
One approach to reducing power requirements in portable computers has been to allow the software to signal that it is waiting for a user input and that the system clock should thus be slowed. When the awaited event such as a keystroke occurs, the software then initiates a speedup in the clock frequency. The speedup can occur without being noticed by the user. However, because the slow speed would affect other background tasks, such as communications, it is generally recommended that the speed of the processor not be slowed down when the system is coupled to a network. This is generally not a problem since, when coupled to a network, a system can usually be plugged into a wall outlet. In fact, some systems only allow the clock slowdown when power is supplied from the battery and it is thus assumed that the system is not coupled to a network.